Lesley Page, visiting professor in midwifery at King's College London, believes that such units improve the birth experience.
The move to have all women give birth in hospital was one of the biggest uncontrolled medical and social experiments of the 20th century, she writes. From 1954 to the 1980s in the UK the percentage of births at home fell from about 35% to 1% in the belief that this would increase safety and reduce the inequalities of care. But this was never evaluated and has never been proved, she says.
The shift has also resulted in a tendency to dehumanisation, particularly in large hospitals, and difficulty in providing personal care appropriate to individual needs.
She believes that birth centres not only provide a further choice for women but also clinical environments where midwives can fully use their skills and provide support for normal birth avoiding necessary intervention.
A one size fits all approach to maternity care is neither advisable nor sustainable, she says. A network of services is required so that women may be referred and transferred when necessary and cared for by the appropriate professional.
Consultant obstetricians have valuable skills that need to be concentrated on the care of women with complicated pregnancies. Safer maternity services are those that recognise and respond to the effects of inequalities and ethnicity, recognise the risk of unnecessary interventions, and support all professionals to play their full part in care, she concludes.
But James Drife, professor of obstetrics and gynaecology at the University of Leeds, remains worried about the risks of delivering outside hospital.
The NHS, which has a near monopoly of childbirth, is promoting midwife units as a way of offering choice and is advising women that they are safe for low risk pregnancies, he writes. But this advice is not based on evidence.
Maternal complications during childbirth are no less frequent than they were in the past, he says, but prompt treatment saves lives every day across the UK, and national maternal mortality is low because emergencies are managed effectively.
However, evidence on safety of midwife led units is lacking. Two reviews suggested trends towards higher perinatal death and, although not statistically significant, they should worry those who want to change patterns of care, he warns. Even in hospitals that have a consultant unit and a midwife led unit in the same building, the evidence is not entirely reassuring.
It is disturbing that in an era of evidence based medicine, midwife led units are being promoted before their safety has been established, he says. The attractions of a relaxed environment and non-intervention are easy to understand, but most women put the highest premium on safety for their baby.
Last year the National Perinatal Epidemiology Unit began an evaluation of alternative locations for labour and birth. Further changes should await reliable evidence on safety and must not be driven by political expediency, he concludes.